Wireless, radio frequency (“RF”) communication systems enable people to communicate with one another over long distances without having to access landline-connected devices such as conventional telephones. In a typical cellular telecommunications network (e.g., mobile phone network), an area of land covered by the network is geographically divided into a number of cells or sectors, which are typically contiguous and which together define the coverage area of the network. Each cell is served by a base station, which includes one or more fixed/stationary transceivers and antennae for wireless communications with a set of distributed wireless units (e.g., mobile phones) that provide service to the network's end-user subscribers. The base stations are in turn connected (e.g., by way of an optical backhaul line) to a mobile switching center (“MSC”) and/or radio network controller (“RNC”), which serve a particular number of base stations depending on network capacity and configuration. The MSC and RNC act as the interface between the wireless/radio end of the network and a public switched telephone network or other network(s) such as the Internet, including performing the signaling functions necessary to establish calls or other data transfer to and from the wireless units.
Various methods exist for carrying out RF communications between the base stations and wireless units. Examples include CDMA (code division multiple access), TDMA (time division multiple access), and OFDM (orthogonal frequency-division multiplexing). CDMA, widely implemented in wireless networks in the United States, is a spread-spectrum multiplexing scheme wherein transmissions from wireless units to base stations are across a single frequency bandwidth known as the reverse link, e.g., a 1.25 MHz (or greater) bandwidth centered at a first designated frequency. Generally, each wireless unit is allocated the entire bandwidth all of the time, with the signals from individual wireless units being differentiated from one another using an encoding scheme. Transmissions from base stations to wireless units are across a similar frequency bandwidth (e.g., a 1.25 MHz or greater bandwidth centered at a second designated frequency) known as the forward link. (The forward and reverse links are also known in the industry as the downlink and uplink, respectively.) In TDMA-based systems, which are widely used in Europe and elsewhere, frequency channels are divided into time slots for sharing among a plurality of users, such that the information for each user occupies a separate time slot of the frequency channel. In OFDM-based communications, the available RF bandwidth is divided into several sub-channels. The bit stream to be transmitted is split into a plurality of parallel, low-rate bit streams. Each bit stream is transmitted over one of the sub-channels by modulating a sub-carrier using a standard modulation scheme.
Regardless of its type, each wireless network has a government-assigned frequency spectrum for supporting RF communications between the wireless units and base stations. Because of this limited bandwidth, and because the demand on this bandwidth increases as the number of wireless users increases, it is desirable in a wireless system to use the available frequency spectrum in the most efficient manner possible. In particular, given a set bandwidth, greater efficiency generally corresponds to a greater network capacity in terms of the number of possible concurrent users and/or data throughput.
A major factor limiting RF efficiency and capacity in broadband wireless networks is intercell interference. Simply put, intercell interference is RF interference as seen in a particular network cell, which originates from transmissions in adjacent cells. Because cellular networks utilize a plurality of transmission sources (e.g., base stations), and because the transmission sources are typically spaced a relatively short distance apart, a wireless unit in a given cell will not only receive transmissions from the base station in its cell, but also transmissions from base stations in neighboring cells. The transmissions received from neighboring base stations are perceived as interference by the receiving wireless unit.